<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="lib/jquery-3.3.1.min.js"></script>
</head>
<body>
    <div id="parent">
        <p>hello world</p>
        <div>
            <p>hello world</p>
        </div>
    </div>
    <button id="btn">btn</button>
    <script>
    $("#btn").click(function(){
        $("#parent>div").css({color:"red"})
    })
    </script>
</body>
</html>